Substrate heating device and substrate heating method

ABSTRACT

A device for heating a substrate with light from a flash lamp having a semiconductor switch connected in series to the flash lamp. After triggering of a trigger electrode of the flash lamp, a first drive signal and a second drive signal are output from a gate circuit. The time period when the semiconductor switch is on due to the second drive signal is longer than the time period that the semiconductor switch is on by the first drive signal. Then, the semiconductor switch is switched on and off by the first drive signal and the substrate temperature is increased to a temperature, which is lower than the desired temperature to be achieved, and is maintained a that temperature for a short time, after which the surface temperature of the substrate is increased to the desired target temperature.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a heating device used in the productionprocess for semiconductors and thin-film transistors, and relates to aflash lamp heating device using a flash lamp as a heating source.

2. Description of Related Art

Conventionally, in order to inject ions onto a most-surface of asubstrate, such as a semiconductor wafer, or to activate a substrate,the substrate is rapidly heated, and consequently, a device for heatinga substrate using a flash lamp is well known (see, Japanese Laid-OpenPatent Application No. 2002-198322 and Japanese Laid-Open PatentApplication No. 2001-319887 (corresponding to US 2002-0179589 A)).

Further, a device for heating a substrate by light from both surfaces,wherein background heating (preheating) is conducted using a halogenlamp, and then, the substrate is rapidly heated with a flash lamp to thetemperature for activating the substrate is also well known (see,International Patent Application Publication No. WO 03/085343).

The flash lamp is a lamp where luminescent gas, for example, xenon (Xe),is enclosed in the sealed inside of a rod-shaped luminous tube, forexample, made from silica glass, and a pair of electrodes are arrangedby facing each other inside the rod-shaped luminous tube. A rod-shapedconductor, for example, made from stainless steel is placed on the outersurface of the luminous tube of the flash lamp along a longitudinaldirection of the luminous tube as a trigger electrode. The flash lamp islit by supplying high voltage to the trigger electrode.

FIG. 9 shows an example of a lighting circuit of the conventional flashlamp.

A coil 23 is connected to a high-voltage side 22 of a flash lamp 5 andto ground 24, and a capacitor 26 is connected in parallel to a seriescircuit of the flash lamp 5 and the coil 23. Energy is supplied to theflash lamp from the capacitor 26. Supplying energy to the capacitor 26is started by switching on a switch SW1 arranged at the high-voltageside 22.

In addition, a trigger electrode 52 is placed to illuminate the flashlamp 5, and the trigger electrode 52 is connected to a trigger coil 30.Switching on a switch SW2 and supplying voltage pulses HV to the primaryside of the trigger coil 30 cause the application of high voltage to thetrigger electrode 52, and the flash lamp 5 is lit.

Lower power, more compact semiconductor integrated circuits have madethe transistor circuit produced within the circuit itself to become avery micro-fabricated circuit. Specifically, it is necessary to reducethe depth of a diffusion layer of impurity atoms contained in thesemiconductor layer for forming the source and drain at both sides ofthe gate in the transistor circuit. On the other hand, the surfaceresistance value (Ω/cm²) of the semiconductor circuit needs to belowered.

The depth of the diffusion layer of the impurity atoms in the transistorcircuit formed on the semiconductor wafer can be reduced by lowering thediffusion temperature or shortening the time during doping of impurityatoms in a diffusion process to dope and diffuse impurity atoms on thesemiconductor wafer.

On the other hand, in the activation process to activate the impuritydiffusion layer and to lower the surface resistance value (Ω/cm²),impurities (dopant) to be diffused on the semiconductor wafer arepositioned not in alignment to the silicon crystal lattice locationafter the diffusion process; however, the activation is completed whenthe dopant itself finds the closest crystal lattice and returns to aproper position. This phenomenon requires only such a short time asapproximately 10 nano-seconds.

Achieving both the high activation and the lower diffusion is realizedby rising the temperature as much as possible and conducting a thermaltreatment in a short time.

For example, if the material used for the semiconductor wafer issilicon, it should be heated at around 1,400° C., which is a temperaturesufficient to melt silicon, for a moment.

As one example, a case of a silicon wafer using boron as a dopant isshown. The silicon wafer needs to be heated at 1,000° C. or higher for1.5 seconds or longer, when the silicon wafer is heated by theconventional spike RTA (optical rapid thermal annealing using a halogenlamp), and setting the resistance value to 1,000 Ω/cm². However, heatingat this temperature for this time period will move (diffuse) boron at acertain concentration, which is situated around 10 nm deep beforeheating, to a depth around 30 nm after the spike RTA heating.

On the other hand, if heating with a flash lamp and similarlyirradiating so as to bring the resistance value to 1,000 Ω/cm², boron ata certain concentration situated around 10 nm deep does not excessivelydiffuse in the depth direction even after heating with the flash lamp,and stays around 10 nm. Heating for a short period is necessary in ordernot to diffuse the dopant, and heating with the flash lamp makes thispossible in actuality.

In fact, when the heating time becomes longer and the temperaturethroughout the entire semiconductor wafer rises higher, the dopantdiffuses in the depth direction of the semiconductor wafer. However,heating with the flash lamp can prevent excessive ion diffusion.

However, when actually heating a substrate, such as semiconductorwafers, using a flash lamp, the temperature of the substrate risesabruptly by light irradiation, and by the abrupt rise of temperature,there are problems of deformation or cracking by the thermal strainoccurring to the substrate.

As mentioned above, using a flash lamp enables short-time heatingwithout ion diffusion spreading the entire substrate. However, risingthe temperature abruptly for a short time causes problems, such asdeformation or cracking due to the thermal strain resulting from thedifference in temperature on the surface and the bottom of thesubstrate.

SUMMARY OF THE INVENTION

The present invention is directed to the solving of problems of the typedescribed above. In particular, the present invention has an object ofproviding a substrate heating device that can activate a topmost surfaceof the substrate while suppressing distortion or breaking of thesubstrate when rapidly heating the substrate using a flash lamp,implanting ions and activating the topmost surface of the substrate.

After considering many ways to solve the problems mentioned above, ithas been discovered that deformation or cracking in a substrate can bereduced by not increasing the surface temperature of the substrate to adesired temperature to be achieved at once, but by increasing thesubstrate temperature up to a second temperature, which is lower thanthe desired temperature, and keeping the substrate temperature at thattemperature for a short time, or by increasing the temperature at acontrolled temperature rising rate, and then, increasing the surfacetemperature of the substrate to the desired temperature to be achieved.

Herein, as described above, if the heating time is long and thetemperature is high throughout the entire substrate, ion diffusion willspread across the entire substrate. Therefore, in order to have adifference in temperature between the surface and the bottom of thesubstrate which does not cause ion diffusion to spread throughout theentire substrate, the retention time of the second temperature issufficiently shortened, or the rising time of the temperature isadjusted to be sufficiently short, and in addition, the temperaturedifference between the surface and the bottom of the substrate isadjusted to be within a temperature difference that will not causedistortion or cracking due to thermal strain.

Based upon the descriptions above, the present invention solves theproblem as follows:

(1) A substrate heating device for heating a substrate by a lamp heatingdevice, comprises: a power source; a capacitor charged by the powersource; a flash lamp that discharges by electrical charges accumulatedin the capacitor; an inductance connected between the capacitor and theflash lamp; a trigger unit causing the flash lamp to start discharging,wherein a diode is connected in parallel to a series circuit comprisingthe flash lamp and the inductance, and a semiconductor switch isconnected in series to the flash lamp.

Further, a drive circuit for outputting a first drive signal that causesthe semiconductor switch to turn on and off at least once after atrigger signal is input into the trigger unit, and for outputting asecond drive signal causing the semiconductor switch to turn on onlyonce after the first drive signal is output is established.

Then, a time period when the second drive signal causes thesemiconductor switch to turn on is longer than that when a drive signalwithin the first drive signals causes the semiconductor switch to turnon, and the flash lamp is lit by switching on and off the semiconductorswitch by the first drive signal, and the substrate temperature isincreased to the second temperature, which is lower than the desiredtemperature to be achieved, and maintained at the temperature for ashort time, or the temperature is increased while the temperatureelevation rate is controlled, and then, the semiconductor switch isswitched on by the second drive signal and the substrate temperature ofthe substrate is increased to the desired temperature to be achieved.

(2) In the above-mentioned solution (1), a second heating device isarranged on the other side of the flash lamp, relative to the substratethat is heated by the lamp heating device.

(3) In the above-mentioned solutions (1) and (2), the first drive signalis an on-off signal where an ON signal causes the semiconductor switchto remain in the on-state and an off signal causes the off-state toalternately appear more than once.

(4) In the above-mentioned solution (3), a duty ratio [period of onsignal/(period of on signal+period of off signal)] of the on-off signalis changed during the period when the first drive signal is output.

(5) In the above-mentioned solutions (1), (2) and (3), an insulated gatebipolar transistor (IGBT element) is used as the semiconductor switch.

The present invention can provide the following effects:

(1) The first drive signal that switches on and off the semiconductorswitch connected in series to the flash lamp at least once and thesecond drive signal that switches on the semiconductor switch only onceafter the first drive signal is output, the on-period time of thesemiconductor switch by the second drive signal is adjusted so as to belonger than the on-period time of the semiconductor switch by one of thefirst drive signals, then the first drive signal switches on and off thesemiconductor switch and the flash lamp is lit, and the temperature ofthe substrate is increased to the second level, which is lower than thedesired temperature to be achieved, or increased while controlling therising rate of temperature, and then, the semiconductor switch isswitched on by the second drive signal, and the temperature is increasedto the desired temperature to be achieved, so the strain due to thetemperature difference in the thickness direction of the substrate to beheated can be minimized, and deformation or cracking can be suppressed.

In other words, extending the on-period time of the semiconductor switchby the second drive signal compared to the on-period time of thesemiconductor by one of the first drive signals can increase thetemperature rising rate by the second drive signal compared to that bythe first drive signal, and thermal damage from theelevating-temperature of the substrate can be reduced, and deformationand cracking can be suppressed.

Further, sufficient shortening of the heating time by the first drivesignal and controlling of the temperature difference between the surfaceand the bottom of the substrate in order not to spread the ion diffusionthroughout the entire substrate enable the prevention of the iondiffusion from spreading throughout the entire substrate.

(2) When heating the substrate, placing a second heating device on theopposite side of the flash lamp relative to the substrate enablespre-heating of the substrate without blocking the light irradiation ofthe flash lamp. Therefore, it enables the input power to the flash lampfor heating the substrate up to the desired temperature to be reduced,and reduces a burden on the flash lamp so that the flash lamp can lastlonger. Further, pre-heating the substrate by resistance heating enablesthe reduction of the temperature difference in the thickness directionof the substrate and suppression of deformation and cracking of thesubstrate.

(3) The first drive signal is an on-off signal where an ON signalcausing the semiconductor switch to keep in the ON state and an OFFsignal causing the switch to stay in the OFF state alternate more thanonce, and thus makes it possible to set the temperature rise rate by thefirst drive signal as desired by changing the duty ratio of the on-offsignal [period of ON signal/(period of ON signal+period of OFF signal)]during the period.

Further, the duty ratio of the on-off signal is changed within a timeperiod while the first drive signal is output, and for example,gradually raising the duty ratio enables the temperature to rise at thegiven rising rate, so that heat damage to the substrate at the time ofraising the temperature can be reduced, and deformation and cracking canbe suppressed.

(4) Using an insulated gate bipolar transistor (IGBT element) as thesemiconductor switch enables the discharging current to be switched in apulse manner even with a flash lamp that requires a large current.Consequently, luminescence of the flash lamp can be controlled to adesired degree by consuming the energy accumulated in the capacitorwhile it is controlled.

The substrate heating device of the present invention, on the occasionof rapidly heating the semiconductor wafer, uses a lighting circuit inwhich a diode is connected in parallel to a series circuit of the flashlamp and an inductance, and the semiconductor switch is connected inseries to the flash lamp, and when the flash lamp is lit, the substrateheating device outputs the first drive signal that switches on and offthe semiconductor switch at least once, and the second drive signalswitches on the semiconductor switch at least once after the first drivesignal is output, and whose period when the semiconductor switch is ONis longer than one of the first drive signals.

With these outputs, in the temperature distribution in the thicknessdirection of the substrate heated using the first drive signal and thesecond drive signal, the temperature difference becomes smaller and thethermal strain experienced by the semiconductor wafer can be reduced, ascompared to the case of heating the flash lamp with a single lightpulse. Thus, the present invention has a great effect in suppressingdeformation and cracking in the semiconductor wafer.

Hereafter, specific embodiment will be explained with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a & 1 b are schematic diagrams showing side sectional and topplan views, respectively, of the configuration of a heating substratedevice of the present invention.

FIG. 2 is a diagram showing one example of the flash lamp lightingcircuit for use in accordance with the present invention.

FIG. 3 is a diagram showing an example of a flash lamp lighting circuitfor illuminating a plurality of flash lamps.

FIGS. 4 a & 4 b illustrate heating of a semiconductor wafer with aheating device and 4 c & 4 d are graphs showing a comparison of thetemperature state in the thickness direction between the case of heatinga semiconductor wafer with a heating device using a flash lamp and inthe case of increasing the temperature with a flash of a single lightpulse.

FIG. 5 is a graph for explaining the timing of the elevating temperaturepattern of the first embodiment of the present invention.

FIG. 6 is a graph of a second embodiment showing the timing of anotherelevating temperature pattern in accordance with the present invention.

FIG. 7 is a graph showing the timing of the elevating temperaturepattern of a third embodiment of present invention.

FIG. 8 is a graph showing the timing the elevating temperature patternin accordance with a fourth embodiment of the present invention.

FIG. 9 is a diagram showing one example of the conventional lightingcircuit for a flash lamp.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a substrate heating unit 1 in accordance with the presentinvention in which a semiconductor wafer (substrate) 3 is arranged on ahot plate 2, and a light irradiating part 4 is arranged above the uppersurface of the semiconductor wafer 3. The light irradiating part 4 iscomposed of a plurality of straight tube-shaped flash lamps 5 arrangedin parallel to a reflecting mirror 6 for reflecting light from the flashlamps 5 toward the facing side of the semiconductor wafer 3. A triggerelectrode 52 is mounted to the flash lamps 5 at the side of thereflecting mirror 6, i.e., the side facing away from the facing side ofthe semiconductor wafer 3.

FIG. 2 shows one example of a flash lamp lighting circuit 21 forrealizing the present invention. A coil 23 and an IGBT element 25 as thesemiconductor switch are connected in series to the flash lamp 5 at thehigh voltage side 22 and a ground side 24, respectively.

Further, a capacitor 26 for supplying energy to the flash lamp 5, and adiode 27 for controlling a feedback current associated with opening andclosing of a gate 251 of the insulated gate bipolar transistor (IGBTelement) 25 are connected in parallel to the flash lamp 5, respectively.Supplying energy to the capacitor 26 is started by switching on theswitch SW1 arranged at the high voltage side 22.

Further, a gate circuit 28 is placed in the IGBT element 25 and controlsthe current flowing into the flash lamp 5 by switching on and off thegate 251 in accordance with a gate signal 281 to be input externally.

In addition, the trigger electrode 52 for illuminating the flash lamp 5is placed on the lamp 5, and the trigger electrode 52 is connected to atrigger coil 30. Voltage pulses are supplied to a primary side 301 ofthe trigger coil 30 in accordance with a trigger signal 302 of the flashlamp 5. This action is started by switching on the switch SW2.

FIG. 3 shows an example of a lighting circuit for operating a pluralityof flash lamps.

As shown in FIG. 3, a diode 27 is connected in parallel to a seriescircuit of flash lamps 5-1 to 5-n and the coils 23, respectively, and aconnecting point of the coil 23 and the diode 27 is commonly connectedvia a diode 31, and is connected to the + side of the power source viathe switch SW1.

Further, the connecting points of the flash lamps 5-1 to 5-n and thediode 27 are commonly connected to one of terminals of the IGBT element25, the other terminal of the IGBT element 25 is connected to the ground24 of the power source, and a gate circuit 28 is connected to the gateterminal of the IGBT element 25.

In addition, the trigger electrode 52 of the flash lamps 5-1 to 5-n areconnected respectively to trigger coils 30, and the primary side 301 ofeach trigger coil 30 is commonly connected, and connected to the switchSW2.

In the case of configuring a plurality of lighting circuits of the flashlamps, as shown in FIG. 3, lighting circuits 21-1 to 21-n are connectedto the flash lamps, respectively, and for example, the switches SW1 andSW2, the IGBT element 25 and the gate circuit 28 can be commonly used.Thus, commonly using the switches SW1 and SW2 and the IGBT element 25enables minimization of variation in illumination timing of the flashlamps.

FIGS. 4 a-4 d show a comparison of the temperature state in thethickness direction between the case of heating a semiconductor waferwith a heating device using the flash lamps and the case of increasingthe temperature with a single pulse flash light.

FIG. 4( a) is a similar diagram as that shown in FIG. 1, and is aschematic depiction of the heating device 1 having the semiconductorwafer 3 arranged on the hot plate 2 and the light irradiating part 4where a plurality of flash lamps 5 are arranged on the surface side ofthe semiconductor wafer 3. The heating device 1 uses a reflecting mirror6 to reflect light emitted from the flash lamp 5 toward thesemiconductor wafer 3, and each flash lamp 5 has a trigger electrode 52on the side facing the reflecting mirror 6.

FIG. 4( b) is an enlarged diagram of a portion n surrounded with abroken line in FIG. 4( a), showing a portion of the semiconductor wafer3 and the hot plate 2. Further, points A, B and C are placed as apattern in the thickness direction of the semiconductor wafer 3, Arepresenting the topmost surface, B an intermediate area and C a rearsurface, respectively. Furthermore, the topmost surface as used hereinrefers to a portion at the light irradiation surface side of thesubstrate wafer of from 3 to 10 μm in depth, and the intermediate areaindicates a portion from 10 μm up to 100 μm. Further, the rear surfaceindicates the surface making contact with the hot plate 2.

FIG. 4( c) shows the relationship between the temperature and time atthe points A, B and C shown in FIG. 4( b). The horizontal axis indicatestime, and the points of time a, c, d and e represents the followingtiming, respectively:

In particular, a is a point of time to start increasing the temperatureof the semiconductor wafer 3; c is a point of time when a trigger signalof the flash lamp 5 is input; d is a point of time when a gate signal ofthe IGBT element 25 for increasing the temperature of the semiconductorwafer 3 to the highest temperature is turned on; and e is a point oftime when the temperature is dropped to the one before illuminating theflash lamp.

On the topmost surface of the semiconductor wafer 3 (the line Aindicated with a solid line), the temperature rises to 500° C. at thepoint c, to 800° C. at the point d, and then, reaches the highesttemperature of 1,300° C., and then lowers to 500° C. at the point e.

Next, on the intermediate level of the semiconductor wafer 3 (at thethickness depth of 100 μm: shown with a broken line B), whenillumination of the flash lamp 5 is started, the temperature starts torise at a little slower pace than the line A, and reaches 700° C. at thepoint d, then reaches the highest temperature of 1,000° C., and thendrops to 500° C. at the point e. Next, on the rear surface of thesemiconductor wafer (shown as chain double-dashed line C), whenillumination of the flash lamp 5 is started, the start of thetemperature increase is greatly delayed relative to that represented bythe solid line A and the broken line B. Then, the temperature risesmoderately even at the point of time d, and only reaches a temperatureof 550° C., after which it gradually declines.

Furthermore, in FIG. 4( c), the periods from the point of time a to thepoint of time c, from the point c to the point of time d, and the pointof time d to the point of time e are indicated with the same length;however, the period from the point of time a to the point of time c is,for example, approximately 3 minutes; from the point of time c to thepoint of time d is, for example, approximately 0.1 seconds; and from thepoint of time d to the point of time e is, for example, approximately0.01 seconds.

FIG. 4( d) shows a comparative example relative to the presentinvention, and a case where energy accumulated in a capacitor issupplied to the flash lamp 5 at once as in the prior art, and one pulseof light is irradiated.

The points A, B and C on the semiconductor wafer 3 shown in FIG. 4( d)correspond to the points shown in FIG. 4( b), respectively, and pointsof time a, f and e indicate the timing mentioned below, respectively.

In particular, a is a point of time at which increasing of thetemperature of the semiconductor wafer 3 starts; f is a point of timewhen a trigger signal is input into the flash lamp 5 after electriccharges are accumulated in the capacitor (the flash lamp 5 starts toemit light from this point of time); and e is a point of time when thetemperature drops back to that at the time that the flash lamp is lit.

The temperature of the points A, B and C at the point of time f on thesemiconductor wafer 3 in this comparative example is 500° C. Afterwards,the flash lamp 5 starts to emit light, and then, the temperature reachesthe highest temperature: 1,300° C. at the point A, 900° C. at the pointB and 520° C. at the point C, and then, drops back to 500° C. at pointof time e.

A comparison of FIG. 4( c) and FIG. 4( d) reveals that both in FIGS. 4(c) and 4(d), the temperature achieved at the point A, which is a thetopmost surface of the semiconductor wafer 3, is set to increase up to1,300° C., and the temperatures achieved at the point B and the point Care different in FIG. 4( c) and FIG. 4( d). In other words, for example,the temperature achieved at the point B is 1,000° C. in FIG. 4( c) whileit is 900° C. in FIG. 4( d).

With such a temperature distribution, at the point A, which is at thetopmost surface, stretching due to heat expansion of the semiconductorwafer 3 occurs according to the temperature of 1,300° C. On the otherhand, at the point B, which is at an intermediate level, the temperatureis lower, and a difference in heat expansion occurs as compared to theexpansion at the topmost surface.

This difference in the heat expansion occurs within a very short timewhen the flash lamp 5 is lit, and the difference in the heatingexpansion associated with a great difference in temperature in the depthdirection of the semiconductor wafer 3 leads to the stress that isexperienced by the semiconductor wafer 3.

In the case of FIG. 4( c) of the present invention, as compared to FIG.4( d) (the conventional temperature-rising method), the difference intemperature is smaller by 100° C., and the stress occurring in thesemiconductor wafer 3 is small. According to this phenomenon, it appearsthat distortion and cracking in the semiconductor wafer 3 can besuppressed.

In order to minimize the difference in temperature between the point A,which is at the topmost surface of the semiconductor 3, and the point B,which is at an intermediate level, a drive circuit for outputting afirst drive signal that switches on and off the semiconductor switch(IGBT element 25), and a second drive signal that switches on thesemiconductor switch after the first drive signal is output is used, andthe time period when the semiconductor switch is ON by the second drivesignal is designed to be longer than the time period when thesemiconductor switch is ON by one of the first drive signals, and whenthe flash lamp 5 arranged in the heating device is lit, the flash lamp 5is lit by switching on and off the semiconductor switch by the firstdrive signal, and the substrate temperature is increased to the secondlevel, which is lower than the desired temperature to be achieved, andthen the semiconductor switch is switched on by the second drive signalto increase the temperature of the topmost surface of the substrate upto the desired temperature to be achieved (1,300° C. in thisembodiment).

Thus, when the flash lamp 5 is lit, the semiconductor switch is switchedon and off, for example, with on-off signals with a predetermined dutycycle, and the current flowing into the flash lamps 5 is limited. As aresult, the temperature gradient in the depth direction thesemiconductor wafer 3 can be moderated.

Herein, it is desirable to conduct a treatment to increase thetemperature of the semiconductor wafer 3 within a short time asdescribed above, and the time to increase the temperature and thetreatment temperature shall be decided appropriately, according to thetype of the semiconductor wafer 3 and the treatment (for example, depthof ion induction treatment on a surface, and a heat damage to alaminated thin layer).

FIG. 5 is a graph of the timing of the temperature-rising pattern in thefirst embodiment of the present invention, and shows a case where thefirst drive signal is composed of a drive signal that allows the firstdrive signal to be ON, for example, for 40 μsec, for the purpose offorming an arc discharge throughout the entire tube axis direction, andanother drive signal that repeats on and off with a certain duty cycle,the second drive signal being composed of an on-signal that continuesfor a certain period of time.

The graph of FIG. 5 shows, respectively from the top: (I) therelationship between temperature and time of the topmost surface of thesemiconductor wafer 3; (II) the gate signal to be input into the IGBTelement; (III) the input timing of the trigger signal for illuminatingthe flash lamp 5; (IV) the charge-initiating signal to the chargingcapacitor to supply power to the flash lamp 5; (V) the lamp voltage ofthe flash lamp 5; and (VI) the lamp current of the flash lamp 5. Thehorizontal axis indicates the time, and the vertical axis indicates thetemperature from room temperature up to 1,300° C.

In FIG. 5, if the time is shown at the horizontal axis, heating to thesemiconductor wafer 3 starts at a certain point of time a. Thisindicates a point of time when the hot plate 2 is arranged as aresistance heating apparatus as described above and the semiconductorwafer 3 is placed on the hot plate and the power supply of the hot plate2 is switched on, or a point of when the semiconductor wafer 3 itself isplaced on the pre-heated hot plate, and the topmost surface temperatureof the semiconductor wafer 3 also starts rising from this point of timea.

After the topmost surface temperature of the semiconductor wafer 3reaches 500° C., the temperature is maintained. When the temperaturemaintenance is started, for example, a charging-initiating signal isissued for starting the charging of the charging capacitor 26 in orderto illuminate the flash lamp 5 at a point of time b (FIG. 5(IV)). Whenthe capacitor charging signal is received, voltage is applied to bothends of the flash lamp 5 (FIG. 5(V)). In this embodiment, for example, avoltage of 4000 V is applied to the flash lamp 5.

Next, at the point of time c, the trigger signal is turned on to startapplying high voltage to the trigger electrode 52 in order to illuminatethe flash lamp 5 (FIG. 5(III)). In association with this, the on-signalto open the gate of the IGBT element 25 connected to the flash lamp 5 isissued (FIG. 5(II)).

The initial pulse of the gate on-signal of the IGBT element is turned onfor 40 μseconds to form the arc discharge of the flash lamp 5 throughoutthe entire tube axis direction of the lamp 5. The gate on-signalfollowing this repeats a cycle of 1 pulse with a 10 μsecond on-periodand an off-period for 10 μseconds more than once. This cycle repeatsuntil the point of time d. During this period (from c to d), thetemperature of the topmost surface of the semiconductor wafer 3 isincreased to 800° C. and maintained.

Further, the lamp voltage of the flash lamp 5 is gradually reducedaccording to the on and off cycle of the gate-on signal. Further, thelamp current flows according to illumination of the flash lamp 5 (FIG.5(VI)).

At the point of time d, the temperature of the semiconductor wafer 3 isincreased to the target temperature by the main discharge of the flashlamp 5. Herein, the gate-on time, for example, is 1 msec, and the energycharged in the capacitor is all discharged. At this point, approximately2,000 A of lamp current flows, and light is emitted from flash lamp 5.This light emitted from the flash lamp 5 causes the rapid increase inthe temperature of the topmost surface of the semiconductor wafer 3 upto 1,300° C.

In this embodiment, the temperature is increased from room temperatureup to 500° C. in a minute, and is maintained at 500° C. for 30 seconds,and then, the flash lamp 5 is lit in accordance with the trigger signal.For this illumination, the gate signal of the IGBT element 25 is turnedon for first 40 μseconds, and then 13 turn-on cycles of 10 μseconds andoff cycles of 10 μseconds were repeated.

At this point (at the point of time after a total of 300 μseconds passedafter the input of the trigger signal), the temperature of the topmostsurface of the semiconductor wafer 3 reaches 800° C. Next, the gatesignal of the IGBT element 25 is turned on for 1 msec, and all of theenergy accumulated in the capacitor is discharged. This causes thetemperature of the topmost surface of the semiconductor wafer 3 to reach1,300° C. After reaching 1,300° C., which is the treatment temperatureof the semiconductor wafer 3, the temperature is lowered down to 500° C.

In this embodiment, 40 μseconds or longer and 100 μseconds or shorterare sufficient for the gate-on signal to open the gate of the IGBTelement 25 to spread the arc to the entire flash lamp 5.

The pulse to be input as the gate signal for the IGBT element 25 shouldbe an on-signal within the range of 10 μsec to 80 μsec, an off-signalwithin the range of 10 μsec to 30 μsec, and a total time of 1 msec to100 msec. Also, for the main discharge of the flash lamp 5, if the gatesignal to be input into the IGBT element 25 is within the range of 0.1msec to 10 msec, 1,300° C. to be achieved can be reached. Furthermore,after the target temperature, 1,300° C., is reached, it drops to 500° C.within 1 msec to 100 msec.

In order to confirm the effect of the present invention, a lightirradiation experiment by the flash lamp 5 to the semiconductor wafer 3was conducted under a condition of the temperature of the topmostsurface of the semiconductor wafer 3 up to 1,500° C. The semiconductorwafer 3 used for the experiment was a Si semiconductor substrate thathad 200 mm diameter, and 725 μm thickness. Further, for the substrateheating device, a device that can uniformly irradiate the semiconductorwafer 3 with 200 mm of a diameter was used. First, light was irradiatedunder the conditions below as the conventional method. The pre-heatingtemperature by the hot plate was 400° C., the on-time of dischargingcurrent flash lamp 5 was 1 ms, and the peak current was 3,000 A.

When five semiconductor wafers 3 were irradiated under the samecondition, three wafers were deformed, and two wafers were cracked. Withthe conventional method of illuminating the flash lamp 5 with thisone-time only increase of the temperature, the semiconductor wafer 3 wascertainly deformed or cracked.

Next, light was irradiated to the semiconductor wafer 3 using thepresent invention. As the conditions, the hot plate 2 was preheated at400° C.; as the on-time of current discharge of the flash lamp 5, thegate was on for the first 40 μs using the IGBT element 25 as shown inFIG. 5 to make certain to grow the arc of the flash lamp 5; thenthirteen times of switching on for 10 μs and off for 10 μs wererepeated.

After that, 2,500 A of peak current flowed and the switch was on forapproximately 1 ms. This raised the surface temperature of the topmostsurface of the semiconductor wafer 3 to 1,500° C. After heating fivesemiconductor wafers 3 under this condition, none of the five wafers haddistortion or cracking.

FIG. 6 shows another temperature elevation pattern in accordance with asecond embodiment of the present invention. The parts (I) to (V)correspond to the same parts as in FIG. 5, and show the relationshipbetween temperature and time of the topmost surface of the semiconductorwafer 3, the gate signal to be input to the IGBT element, the inputtiming of the trigger signal, a charging-initiating signal to thecharging capacitor, the lamp voltage and the lamp current, respectively.

FIG. 6 is the same as FIG. 5 until the point of time c when the triggersignal of the flash lamp 5 is input. The pulse width to be input firstas a gate signal of the IGBT element 25 is 40 μsec, similar to FIG. 5.

Subsequently, in this embodiment, until the topmost surface temperatureof the semiconductor wafer 3 reaches 800° C., the IGBT element 25 isdriven by the first drive signal, and the lamp is lit by switching thegate signal on and off for 10 μsec each. In addition, after that, thelamp is lit by switching on the gate signal for 20 μsec and off for 10μsec, and the surface temperature of the topmost surface of thesemiconductor wafer 3 is increased to 1,050° C.

Next, the IGBT element 25 is driven by the second drive signal, andon-time of the gate is extended to 1 ms to discharge all energyaccumulated in the capacitor for discharging the flash lamp 5, and thesurface temperature of the topmost surface of the semiconductor wafer 3is increased to 1,300° C. Then, the temperature is lowered to 500° C.,which is the temperature for pre-heating by the hot plate. As in thisembodiment, changing the pattern of the gate signal of the IGBT element25 enables control of the pattern of elevating-temperature of thetopmost surface temperature of the semiconductor wafer 3.

FIG. 7 shows another temperature elevation pattern in accordance with athird embodiment of the present invention, and parts (I) to (VI) are thesame as in FIG. 5, indicating the relationship between temperature andtime of the topmost surface of the semiconductor wafer 3, the gatesignal to be input to the IGBT element, the input timing of the triggersignal, a charging-initiating signal to the charging capacitor, the lampvoltage and the lamp current, respectively.

FIG. 7 is also the same as FIG. 5 until the point of time c when thetrigger signal of the flash lamp 5 is input.

In addition, the IGBT element 25 is driven by the first drive signal,and in this embodiment, the pulse width to be input first as a gatesignal of the IGBT element 25 is 40 μsec as similar to FIGS. 5 & 6.However, switching the gate signal of the IGBT element 25 to be input isturned on for 10 μsec, off for 10 μsec, on for 20 μsec and off for 10μsec, and this enables the surface temperature of the topmost surface ofthe semiconductor wafer 3 to increase to 800° C. in a shorter time.

After that, switching on the gate signal of the IGBT element 25 for 20μsec and the following gate signal off for 20 μsec raises the surfacetemperature of the topmost surface of the semiconductor wafer 3 up to1,000° C. In addition, after an off-time of 10 μsec, the IGBT elementsignal 25 is driven by the second drive signal, and on-time of the gateis extended to 1 ms. With this operation, all energy accumulated in thecapacitor is discharged for discharging electricity of the flash lamp 5,and the topmost surface temperature of the semiconductor wafer 3 isincreased to 1,300° C. Then, the temperature is lowered to 500° C.,which is the temperature of pre-heating by the hot plate. Thus, changingthe on-time of the gate signal of the IGBT element 25 enables thecontrol of the pattern of elevating temperature of the topmost surfacetemperature of the semiconductor wafer 3.

FIG. 8 shows another temperature elevation pattern in accordance with afourth embodiment of the present invention, and parts (I) to (VI) arethe same as in FIG. 5, showing the relationship between temperature andtime of the topmost surface of the semiconductor wafer 3, the gatesignal to be input to the IGBT element, the input timing of the triggersignal, a charging-initiating signal to the charging capacitor, the lampvoltage and the lamp current, respectively.

FIG. 8 is also the same as FIG. 5 until the point of time c when thetrigger signal of the flash lamp 5 is input.

In this embodiment, the first drive signal sent to the IGBT element 25is a gate signal with a single pulse. That is, switching the gate signalof the IGBT element 25 on for 110 μs causes the increase of the surfacetemperature of the topmost surface of the semiconductor wafer 3 to 900°C.

In addition, after the off-time for 10 μsec, the IGBT element 25 isdriven by the second drive signal, and the on-time of the gate isextended to 1 ms. This results in the discharge of all energyaccumulated in the capacitor for illuminating of the flash lamp 5, andthe topmost surface temperature of the semiconductor wafer 3 isincreased to 1,300° C.

Then, the temperature is lowered to 500° C., which is the pre-heatingtemperature by the hot plate. Thus, changing the on-time of the gatesignal of the IGBT element 25 enables the control of the pattern ofelevating temperature of the topmost surface of the semiconductor wafer3.

1. A substrate heating device for heating a substrate by a lamp heatingdevice, comprising: a power source; a capacitor charged by the powersource; a flash lamp that discharges by electrical charges accumulatedin the capacitor; an inductance connected between the capacitor and theflash lamp; a trigger unit causing the flash lamp to start discharging,wherein the substrate heating device comprises: a diode connected inparallel to a series circuit composed of the flash lamp and theinductance, a semiconductor switch connected in series to the flashlamp, and a control unit for controlling on/off of the semiconductorswitch; wherein the control unit has a drive circuit for outputting afirst drive signal causing the semiconductor switch to turn on and offat least once after a trigger signal is input into the trigger unit, andfor outputting a second drive signal causing the semiconductor switch toturn on only once after the first drive signal is output; and wherein atime period during which the second drive signal causes thesemiconductor switch to turn on is longer than that a drive signal ofthe first drive signals causes the semiconductor switch to turn on. 2.The substrate heating device according to claim 1, wherein a secondheating device is arranged on an opposite side of the substrate relativeto that at which the flash lamp is located, the second heating devicecomprising a lamp heating device.
 3. The substrate heating deviceaccording to claim 1, wherein the first drive signal is an on-off signalof which an on signal causes the semiconductor switch to keep in theon-state and an off signal causes the off-state to alternately beproduced more than once.
 4. The substrate heating device according toclaim 3, wherein a duty ratio of the on-off signal, defined as theperiod of the on signal divided by the sum of the period of on signalplus the period of the off signal, is changed during the time when thefirst drive signal is output.
 5. The substrate heating device accordingto claim 1, wherein the semiconductor switch is an insulated gatebipolar transistor.
 6. The substrate heating device according to claim1, wherein a plurality of flash lamps is provided.
 7. The substrateheating device according to claim 6, wherein the flash lamps arestraight tube-shaped flash lamps arranged in parallel to a reflectingmirror for reflecting light from the flash lamps toward the substrate.8. A substrate heating method, comprising the steps of: switching on asemiconductor switch connected in series to a flash lamp after a triggersignal is received from a trigger device, starting discharge of theflash lamp and emitting light from the flash lamp, irradiating asubstrate so as to cause it to be heated, wherein said switching stepcomprises issuing a first drive signal that switches on and off thesemiconductor switch at least once and a second drive signal thatswitches on the semiconductor switch only once after outputting of thefirst drive signal to the semiconductor switch to illuminate the flashlamp; wherein the substrate temperature is increased by the first drivesignal so as to adjust a temperature difference between a top surfaceand a bottom of the substrate so as to prevent the spread of iondiffusion throughout the entire substrate, and to limit adverse thermalstrain effects on the substrate; and wherein the surface temperature ofthe substrate is increased by the second drive signal to a desiredtemperature which is higher than the temperature at the top surfaceproduced due to the first drive signal.